This invention relates to a static var (volt-ampere reactive) compensator wherein current flowing through a reactor is controlled by static switch means such as thyristors, thereby to compensate reactive power.
FIG. 5 illustrates the principle of a static var compensator. As shown in the figure, an A. C. power source 1 has one end thereof grounded and has the other end thereof connected through a system impedance 4 to a static var compensator (SVC) 5 and also to a fluctuating load 2 such as arc furnance. The system impedance 4 denotes an impedance from the A. C. power source 1 to the node 3 between the SVC 5 and the load 2. A current transformer 6 detects current flow to the load 2, thereby to inform the SVC 5 of the fluctuation of the load 2. The SVC 5 suppresses a voltage fluctation which is incurred at the node 3 by the fluctuation of the load 2.
FIG. 6 is a single-line connection diagram showing a prior-art SVC 5. This SVC 5 comprises a parallel connection assembly having capacitor 7 for supplying reactive power and a reactor 8 for absorbing reactive power. Static switch means 9 such as thyristors, which are connected in inverse parallel relationship, are connected in series with the reactor 8 and control current flowing through the reactor 8. The gate terminals of the static switch means 9 are connected to a controller 10, and the node between the capacitor 7 and the reactor 8 is connected to an output terminal 11.
FIG. 7 is a three-line connection diagram showing the prior-art SVC 5. In the figure, the same symbols denote identical or corresponding portions.
Referring to FIG. 5, when the current flow to the load 2 is denoted by I.sub.A, this load current I.sub.A can be decomposed into an active current I.sub.Ar and a reactive current jI.sub.Ai. EQU I.sub.A =I.sub.Ar +jI.sub.Ai ( 1)
where ##EQU1##
Assuming now that the SVC 5 be not operating, so current I.sub.SVC =0 hold, then the voltage VA at the node 3 undergoes a voltage drop (.DELTA.V) by the product between the system impedance 4 having a value Z.sub.S and the load current I.sub.A when compared with the voltage V.sub.S of the A. C. power source 1. EQU .DELTA.V=Z.sub.S .times.I.sub.A ( 2)
Z.sub.S includes, the impedances of a transmission line or a transformer, which is substantially equal to an inductance component jX.sub.S. EQU Z.sub.S .div.jX.sub.S ( 3)
Accordingly, the magnitude .DELTA.V of the voltage drop is expressed by the product between the inductance component jX.sub.S of the system impedance 4 and the reactive current jI.sub.Ai in the current I.sub.A flowing to the load 2. EQU .DELTA.V=jX.sub.S .times.jI.sub.Ai =-X.sub.S .multidot.I.sub.Ai ( 4)
That is, the voltage V.sub.A at the node 3 fluctuates according to the reactive current jI.sub.Ai which flows through the load 2. Therefore, when the load fluctuations comprise a large reactive component, for example, as in arc furnaces, the voltage at the node fluctuates greatly, thereby disturbing other power customers.
The SVC 5 is installed in order to suppress such voltage fluctuations. The current I.sub.A flowing to the load 2 is detected by the CT 6, and the SVC 5 provides the reactive current I.sub.SVC in response thereto which cancels the reactive current component in the detected current, thereby to cancel the fluctuation of the reactive current flowing through the system impedance 4 and to stabilize the voltage at the node 3.
Next, the operation of the prior-art SVC shown in FIG. 6 will be described with reference to FIG. 8. The capacitor 7 supplies the output terminal 11 with a fixed reactive power. On the other hand, the reactor 8 consumes the reactive power. Therefore, the SVC 5 supplies the output terminal 11 with the difference between the reactive power supplied by the capacitor 7 and the reactive power consumed by the reactor 8. In this regard, the current flow through the reactor 8 is controlled by the static switch means 9 so as to produce an output reactive power which cancels the reactive power detected by the controller 10. Accordingly, the fluctuation of reactive current generated by the load 2 is canceled to stabilize the voltage at the node 3.
Since the prior-art SVC is constructed as described above, there has been the problem that the capacitor, the reactor and the static switch means need to have large capacities which correspond to the maximum leading reactive capacity to be provided by the SVC, respectively.
This invention has been made in order to eliminate the problem of the prior-art apparatus as described above, and has for its object to reduce the capacities of a capacitor, a reactor and static switch means.